Main Relay (Computer/Fuel System): Testing and Inspection

ECM Voltage Supply, Checking

Special tools, testers and auxiliary items required

• Multimeter.

• Wiring diagram.

Test requirements

• The Engine Control Module (ECM) (J623) fuses OK.

• Battery voltage at least 12.5 volts.

• All electrical consumers such as, lights and rear window defroster, switched off.

• Vehicles with automatic transmission, selector lever in position "P" or "N".

• A/C switched off.

• Ground connections between engine/transmission/chassis OK.

• Ignition switched OFF.

Test procedure

- Perform a preliminary check to verify the customers complaint. Refer to => [ Preliminary Check ] Preliminary Check

Start diagnosis

- Remove the Engine Control Module (ECM) Power Supply Relay (J271).

Checking voltage

- Using a multimeter, check the Motronic Engine Control Module (ECM) Power Supply Relay Box socket 1 (relay terminal 30) to Ground.

Specified value: Battery voltage.

If the specified value was Not obtained:

- Check the wiring connections from the voltage source to the Engine Control Module (ECM) Power Supply Relay (J271) socket 1 for an open or a short circuit. Refer to Wiring Diagrams for circuit detail.

- Check the wiring connections for damage, corrosion, loose or broken terminals.

- If necessary, repair the faulty wiring connection.

If the specified value was obtained:

- Switch the ignition ON.

- Using a multimeter, check the Engine Control Module (ECM) Power Supply Relay Box socket 3, (relay terminal 86) to Ground.

Specified value: Battery voltage.

If the specified value was Not obtained:

- Remove the fuse in Fuse Box B and check resistance of circuit between fuse and relay terminal socket 1 (relay terminal 30). Refer to Wiring Diagrams for pin locations and fuse ID.

Specified value: 1.5 ohm or less.

- If fuse is blown, locate short to ground in circuit, refer to wiring diagram.

- Repair as necessary.

If the specified value was obtained:

Checking activation

- Remove the Engine Control Module (ECM) Power Supply Relay (J271).

- Using a diode test lamp (12 V) connected to Battery + check the Engine Control Module (ECM) Power Supply Relay Box socket 4 (relay terminal 85) while cranking the engine.

The test lamp should be ON.

- Switch the ignition OFF.

If the test lamp was ON:

- Replace the Engine Control Module (ECM) Power Supply Relay (J271):.

If the test lamp was not ON:

- Using a multimeter, check the Engine Control Module (ECM) Power Supply Relay socket 4 (relay terminal 85), to the Engine Control Module (ECM) (J623) electrical harness connector T94 terminal 69 for resistance.

Specified value: 1.5 ohms Max.

If the specified value was Not obtained:

- Check the wiring connection for an open circuit, high resistance or short circuit to Battery (+).

- Check the wiring connection for damage, corrosion, loose or broken terminals.

- If necessary, repair the faulty wiring connection.

If no malfunction is found in the wiring and ground was not present from the ECM during crank:

- Replace the Engine Control Module (ECM) (J623). Refer to the Repair Information.

Final procedures

After the repair work, the following work steps must be performed in the following sequence:

1. Check the DTC memory. Refer to => [ Diagnostic Mode 03 - Read DTC Memory ] Diagnostic Modes 01 - 09.

2. If necessary, erase the DTC memory. Refer to => [ Diagnostic Mode 04 - Erase DTC Memory ] Diagnostic Modes 01 - 09.

3. If the DTC memory was erased, generate readiness code. Refer to => [ Readiness Code ] Monitors, Trips, Drive Cycles and Readiness Codes.